Inkjet printing mechanisms may be used in a variety of different inkjet apparatus, such as plotters, facsimile machines, copiers, and inkjet printers collectively called in the following as printers, to print images using a colorant, referred to generally herein as “ink”. These inkjet printing mechanisms use inkjet cartridges, often called “pens” or “printheads” to shoot drops of ink onto print media, which can be used in the form of cut sheets or rolls of print media.
In the following, for sake of simplicity, with the term “sheet” or “medium” we refer to any generic kind of print media, e.g. paper, vinyl, films, canvas or the like, produced in any form, e.g. cut sheets or rolls, and of any dimensions.
Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as “off-axis” systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located “off-axis” from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multicolour cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a “pen.”
Each pen has a nozzle plate that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporisation chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energised to heat ink within the vaporisation chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energised resistor.
To print an image, the printhead is scanned back and forth across a printzone at a very close distance above the sheet, with the pen shooting drops of ink as it moves. By selectively energising the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the “swath height” of the pen, the maximum pattern of ink which can be laid down in a single pass.
For placing the remaining print swath on the print media known mechanism are then employed to advance or index the medium in the print zone, in a second direction, also called media direction, which is usually substantially perpendicular to scanning direction of the printhead.
U.S. Pat. No. 5,363,129 describes a printing media feed and retaining apparatus which has a plurality of pinch rollers mounted on a single pinch roller support member co-operating with a main drive roller to precisely advance the media in the media direction and control the spacing between the printhead and the surface of the sheet on which printing is to take place.
However, in known printers, when a lot of ink is placed on the sheet in order to print the image, the sheet expands, and this effects are know as media “curl” and “cockle”.
Very often the result of this effect is more problematic near the sheet edges due to the way the deformation occurs. In fact, this expansion may generate at the sheet edge a wave high up to 2-3 mm within the printzone causing the crash of the pen.
The crash of a pen against the medium may seriously affect the print quality or the throughput of the printer due to damages to the pen itself, which can be very persistent or even permanent. In fact it may generate, in the pen, a large number of malfunctioning nozzles which can be hardly replaced with success by working ones to maintain the same print quality or the recovery services of the pen would be repetitively activated to attempt to recover the malfunctioning nozzles.